A Flickering Pressure Reading on a 4-20 mA Loop Near a VFD
A pressure transmitter on a test rig is specified 0 to 10 bar, 4-20 mA, two-wire, loop powered from 24 V DC, and it needs at least 12 V at its own terminals. The signal is read across a 250 ohm precision shunt at the control panel 120 m away; the loop cable has a resistance of 0.15 ohm per metre per core. The shunt voltage is fed to a 12-bit ADC with a 0 to 5 V input range, calibrated to display pressure directly. The cable runs in a tray alongside a variable-frequency drive feeder. During a test the display flickers by about 0.15 bar whenever the drive runs, and on one occasion it showed 0 bar while the rig was still pressurised.
1. For a true pressure of 6.4 bar, compute the loop current, the shunt voltage and the ADC count. Then verify that the loop has enough compliance voltage.
Read the worked answer
Current. A 4-20 mA transmitter maps 0-10 bar onto 16 mA of span. I = 4 + 16 x (6.4 / 10) = 4 + 10.24 = 14.24 mA
Shunt voltage. V = I x R = 0.01424 x 250 = 3.56 V
ADC count. A 12-bit converter has 2^12 = 4096 codes. Counts = (3.56 / 5) x 4095 = 0.712 x 4095 = about 2916 counts
Compliance check. Total loop resistance = shunt 250 ohm + cable (2 cores x 120 m x 0.15) = 250 + 36 = 286 ohm. Maximum permissible loop resistance at 20 mA = (24 V - 12 V) / 0.020 A = 600 ohm. Because 286 ohm is well below 600 ohm, the transmitter still sees more than 12 V at full scale; the loop is electrically valid. Note the margin: at 20 mA the shunt alone drops 5 V, exactly the ADC full scale, so there is no headroom for a series resistor or an added barrier without recalculating the 600 ohm limit.
2. Quantify the resolution in bar per ADC count, and decide whether the 0.15 bar flicker is a resolution limit or an interference problem.
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Resolution, one step at a time.
- One ADC count = 5 V / 4096 = 1.221 mV
- Through the 250 ohm shunt: 1.221 mV / 250 ohm = 4.88 microamp
- Current-to-pressure scaling: 10 bar / 16 mA = 0.625 bar per mA = 0.000625 bar per microamp
- One count therefore represents 4.88 x 0.000625 = 0.00305 bar, about 3 mbar
Verdict. A 0.15 bar flicker is 0.15 / 0.00305 = about 49 counts. That is two orders of magnitude above the quantisation step, so resolution is not the cause and no amount of ADC filtering will explain the problem away. Because the disturbance appears exactly when the variable-frequency drive runs, and the signal cable shares its tray, the cause is induced noise plus a likely earth/return-path problem on the screen or the common of the shunt.
Recommended actions: replace the wiring with shielded twisted pair, earth the screen at the panel end only to avoid a circulating screen current, route the signal cable away from the drive feeder (crossing at right angles where unavoidable), keep the shunt and ADC common referenced at one point, and verify the transmitter's own damping setting. The 4-20 mA current loop itself is inherently immune to series voltage induced in the loop; the noise is being picked up at the receiving end, which is why the shunt wiring deserves the most attention.
3. Explain the 0 bar reading recorded while the rig was pressurised, and state what protection the live-zero signal gives you.
Read the worked answer
A genuine 0 bar condition would give 4 mA, i.e. 1.00 V across the shunt and about 819 counts - not zero. The recorded 0 bar means the ADC saw essentially 0 V, which points to one of:
- Broken or open loop - a loose terminal, a failed shunt, or the loop supply lost. The transmitter cannot drive current through an open circuit, so the display falls to 0.
- Transmitter failure or a blown loop fuse - same symptom, 0 mA.
- Shunt short or ADC input shorted, giving 0 V while current still flows in the loop.
This is the practical value of the 4-20 mA live-zero convention: 0 mA is outside the valid signal range, so the reading is unambiguously a fault, whereas on a 0-5 V system a genuine 0 V and a broken wire look identical. The ADC firmware should flag any current below about 3.6 mA (below roughly 720 counts) as a loop fault rather than displaying it as pressure, and the display should latch the fault.
Diagnostic sequence: clamp-meter the loop current, then measure voltage at the transmitter terminals to confirm the 12 V minimum, then check the shunt and panel terminals. Also verify the drive's earth and the panel earth are bonded to the same reference, since an intermittent open loop can be produced by a loose screen or terminal disturbed by vibration while the drive runs.